JNI UnsatisfiedLinkError没有错误的方法名称和指定的库路径

Question

I'm trying to work build my very first JNI application, following this tutorial: https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html

Problem Summary: While running my application, I get an java.lang.UnsatisfiedLinkError error.

First I wrote the Class HelloJNI.java :

public class HelloJNI {
   static {
      System.loadLibrary("hello"); // Load native library at runtime
                                   // hello.dll (Windows) or libhello.so (Unixes)
   }

   // Declare a native method sayHello() that receives nothing and returns void
   private native void sayHello();

   // Test Driver
   public static void main(String[] args) {
      new HelloJNI().sayHello();  // invoke the native method
   }
}

This class I compiled with: javac HelloJNI.java Next I ran javah HelloJNI This produced the following file HelloJNI.h :

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloJNI */

#ifndef _Included_HelloJNI
#define _Included_HelloJNI
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloJNI
 * Method:    sayHello
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_HelloJNI_sayHello
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

Next I implemented HelloJNI.c :

#include <jni.h>
#include <stdio.h>
#include "HelloJNI.h"

// Implementation of native method sayHello() of HelloJNI class
JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {
   printf("Hello World!\n");
   return;
}

Finally I compiled the c class:

• gcc -I"/usr/lib/jvm/java-8-oracle/include" -I"/usr/lib/jvm/java-8-oracle/include/linux" -c -Wall -Werror -fpic HelloJNI.c
• gcc -shared -o hello.so HelloJNI.o

This produces the files hello.so and HelloJNI.o. Next I try to run the code:

• java -Djava.library.path=. HelloJNI This produces the error:

  Exception in thread "main" java.lang.UnsatisfiedLinkError: no hello in java.library.path at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1864) at java.lang.Runtime.loadLibrary0(Runtime.java:870) at java.lang.System.loadLibrary(System.java:1122) at HelloJNI.(HelloJNI.java:3)

This seems to be the most common JNI error on the internet... My method names seem to be correct. I also ran:

• nm hello.so | grep say

This gives me: 00000000000006b0 T Java_HelloJNI_sayHello which seems to be correct, ie the compiler didn't add additional characters. I simply ran out of ideas of things I could try. Any suggestions?

My OS: Linux Mint 13, GCC version 4.7.3, java version 1.8.0_60

==========UPDATE=============== When I replace System.loadLibrary("hello"); by System.load("/usr0/home/jkinable/workspace/javaIDEA/jnitest/hello.so"); my HelloWorld example works! However, I don't want to use an absolute path so I'm still looking for a way to use System.loadLibrary("hello"); instead? Any suggestions? I've also tried running on a different linux system, but I get the same issue.

Answer 1

It turns out that the problem is due to some naming convention on unix/linux platforms! When using: System.loadLibrary("hello"); the file should not be named hello.so! Instead, the name should be libhello.so . On Windows, use hello.dll. I'm surprised that this issue is not mentioned in IBM's JNI tutorial: http://www.ibm.com/developerworks/java/tutorials/j-jni/j-jni.html

I'm not sure what the rationality behind this issue is. Why would you load a library "hello" which should be named libhello.so on your filesystem (instead of hello.so)?